1. Field of the Invention
This invention relates to the use of perpendicular perforations on a zipper tape for the evacuation of air entrapped in a package, such as a plastic bag.
2. Description of the Prior Art
In the prior art, it is known to provide perforations to provide for a predetermined teat line across sheet media. In order to increase the chances that the resulting tear remains on the tear line, the tear line can comprise colinear line segments oriented in the direction of the perforation. However, if the perforations are desired for some purpose other than providing a tear line, for example to provide for evacuation of air from within a package, the is use of a tear line comprised of colinear line segments oriented in the direction of the perforation is disadvantageous in that this increases the chances of tear propagation along the tear line.
Prior art slit perforating uses a round wheel with raised teeth around its periphery to cut slits which are colinear line segments (see FIG. 1). The teeth are pressed against a hardened anvil roller and a web, or sheet, of material is pulled between the perforating wheel and the anvil roller. Where the tooth comes into contact with the anvil roll a slit (cut) is left in the web. Between the teeth the web is left intact (tie). The lengths of these cuts and ties along with the material being perforated determines the tendency of the web to tear along the perforation. A low ratio of tie area to cut area is illustrated in FIG. 1 and results in a maximized likelihood of tearing. However, in order to minimize the likelihood of tearing, it is necessary to have a high ratio of tie area to cut area. In other words, in order to reduce tear tendency with traditional slit perforating, a relatively large tie area is needed, but this large tie area would result in a low number of slits per lineal inch.
Air evacuation slits are typically very small, such as 0.008 inches, to keep the contents of the bag (such as flour) from being forced out through the slits when the package is compressed to evacuate the air. A large number of slits are required to evacuate air through the very small slits quickly enough so that it does not slow down the automatic packaging equipment.
Moreover, air evacuation apertures for containers made from sheets of plastic web, such as plastic bags, are frequently formed by lasers, which can be expensive.
Prior art references disclosing tear-open elements for carrier stock include U.S. Pat. No. 5,115,910 entitled xe2x80x9cCarrier Stock With Tear-Open Band Segmentsxe2x80x9d issued to Klygis et al. on May 26, 1992 and U.S. Pat. No. 5,020,661 entitled xe2x80x9cCarrier Stock With Tear-Open Tabsxe2x80x9d issued to Marco et al. on Jun. 4, 1991.
It is therefore an object of this invention to provide an apparatus and method for quickly evacuating air from a package.
It is therefore a further object of this invention to provide an apparatus and method to provide a perforation which is tear resistant.
It is therefore a still further object of this invention to provide an apparatus and method to provide a perforation with a high ratio of tie area to cut area.
It is therefore a still further object of this invention to provide an apparatus and method which achieves the above objects at a low price.
These and other objects are attained by providing zipper tape with a perforation comprised of a series of tears which are parallel line segments which are perpendicular (or at least something other than parallel or colinear) to the direction of the perforation. A cutting wheel is used to generate this perforation.